The present invention relates to an exhaust system of a four-stroke internal combustion engine, which is provided with a system for controlling the back pressure in the exhaust port and supercharging pressure. The piston of the engine discharges burned gases against the back pressure in the exhaust stroke, which requires an amount of work. On the other hand, in intake stroke, the piston draws air-fuel mixture into the cylinder through an opening controlled by a throttle valve. The difference between the back pressure and the intake pressure is called pumping losses. In wide-open throttle operation, the intake pressure in the induction passage is relatively high, the back pressure is nearly equal to the atmospheric pressure. Therefore, the pumping losses in wide-open throttle operation are small.
However, in part-open throttle operation, the intake pressure in the induction passage is lower than that in the wide-open throttle operation. Generally, in part-open throttle operation. Therefore, the pumping losses increase in part-open throttle operation, which causes decrease of the thermal efficiency resulting in deterioration of fuel consumption.